Abstract: This article main analysis the dynanic changes characteristics of Jiangchuan digital gas radon since 2004,abtain that the digital gas radon has a year cycle feature,includes two form of changes with the relatively stable and high value changes.On this basis,the original curve method was used in the relatively stable period and the second level of speed method was used in the high-value changes period to extract precursory anomaly feature,and obtained that when the day average gas radon value of relatively stale period C(Rn)+2.0 and the day average gas radon value of high value change C(Rn)+1.8,there will be a better effects of short term forecast to more than 4.0 magnitude earthquake 80km far from the epicenter and to more than 5.0 magnitude earthquake 250km far from the epicenter.